Vein wall structure and mechanical properties will be quantitatively studied and related to the ability of the venous system to control venous blood volume and central venous pressure. Vein wall structure will be determined with an approach combining quantitative image analysis with specific histologic staining of wall smooth muscle, collagen, elastin, and mucopolysaccharides. The content, distribution, and orientation of wall components will be measured. The mechanical properties of vein segments will be determined using photographic measurements in vitro or radiographic measurements in situ. Anatomical variations in the venous system will be used to quantify the relationship between structure and function, particularly constrictive ability. The rate and amount of proliferation of vein smooth muscle in response to increased venous pressure and the consequences of this proliferation will be determined. The effects of changes in axial strain on the volume of venous segments will be studied to determine if this mechanism augments venous return. The ability of venous smooth muscle to augment venous return by increasing the time constant for creep will also be determined. The neurohumoral control of canine vein wall properties will be measured during carotid sinus hypotension and also during hemorrhage. The proposed work will provide new, quantitative information about veins and their ability to reduce their volume during physiologic stress.